Testing of inorganic and organic pigments, plastics, or lacquers with regard to resistance to action of light and weather may involve exposing samples to naturally occurring light and weather conditions. Alternatively, to shorten the test period, exposure may be by means of apparatus for accelerated weathering having a high-intensity radiation source which typically has an enhanced ultraviolet component. In either case it has become apparent that mere correlation of test results to duration of exposure leads to widely scattered results, due mainly to variation in light intensity and spectral composition during sample exposure. In particular, meaningful comparison of test results is often impossible when test apparatus for accelerated weathering of different design are used, especially when different radiation sources are involved such as, e.g., carbon arc, xenon, mercury vapor, or fluorescent lamps.
In known radiation measuring devices various filters may be interposed between a radiation source and a radiation detector, thereby providing means for recognizing (at least qualitatively) changes of the radiation used in one or another of selected narrow spectral ranges. In one such known device, an exchangeable filter can be interposed between a radiation source and a radiation detector. In another, well-known radiation intensity measuring device, three different filters are interposed in consecutive time periods, and the radiation intensities of the three corresponding wavelength intervals are measured in turn. Both devices are suitable only for the determination of radiation from xenon arc lamps.
Such devices are insufficient for several reasons. It is known that the decomposition behavior of binders or pigments is strongly dependent on the spectral composition of incident light. Accordingly, there is a need to expose samples to radiation which is tuned to the photoactive region of such binder and/or pigment. This, in turn, calls for a radiation intensity measuring device which is selectively sensitive to one or several desired spectral regions.
Furthermore, uniformity of sensitivity in a spectral region is not assured in known devices; intensive irradiation tends to cause unnoticed and uncontrollable changes in the properties of bell-jar and interference filters. Such changes lead to considerable uncertainity as to just how much radiation the samples were exposed to and this, in turn, results in the well-known, undesirably wide range of scattering of measured values obtained from test apparatus, especially when measurements of intensity are made in the ultraviolet spectral region (which here is of special interest).
If a device includes only one filter, no detailed information is obtained about intensity as differentiated according to wavelength of radiation incident on a sample. If, however, a device includes three filters, filter changing may preclude reliably taking account of short-term variations of radiation intensity.
In practice, exposure time is often used instead of radiation dosage as a measure of readiation incident on the samples. This method depends on constancy of distribution of radiation and constancy of intensity of the radiation sources (i.e., ambient radiation under natural conditions and artificial radiation under accelerated test conditions); such constancy is usually not assured in practice.